Start SheepShaver and in SheepShaver preferences, Volumes tab, Unix Root: Add the path to your shared folder using the "Browse…" button to find and "Open" the folder you created to be the shared folder. (In the example below I used a folder named "Shared" on my desktop.)

Save the change, quit SheepShaver and launch SheepShaver again. The shared folder will appear as a volume Unix on the OS9 desktop.